1. Field of the Invention
This invention relates to a vibration driven motor or actuator.
2. Related Background Art
FIG. 2 shows polarization patterns and an arrangement of piezoelectric elements (PZT) used in an oscillator (or a vibration member) formed in a rod-like shape in a vibration driven motor (ultrasonic driven motor). Each piezoelectric element PZT is subjected to polarization processing, so that right and left portions on opposite side of a central line (as a boundary) have opposite polarities. These piezoelectric elements are grouped into A and B phase groups each including two piezoelectric elements, and the A and B phase groups are arranged to have a 90.degree. phase difference therebetween. Note that the lowermost piezoelectric element is used for vibration detection (S phase), and although not shown, electrode plates are inserted between each two adjacent piezoelectric elements.
The driving principle will be described below with reference to FIG. 3. When an AC voltage is applied to only the A phase, a primary flexural natural vibration is excited in an oscillator 1 in a direction parallel to the plane of the drawing of FIG. 3 by expansion/shrinkage of the piezoelectric elements. When an AC voltage is applied to only the B phase, a vibration is generated in a direction perpendicular to the plane of the drawing of FIG. 3.
When a vibration in the horizontal direction excited by the A phase and a vibration in the vertical direction excited by the B phase are applied with a 90.degree. temporal phase difference therebetween, a clockwise or counterclockwise motion with respect to the longitudinal axis is generated in the oscillator 1. Since the oscillator 1 has a circumferential groove 1q for amplifying the displacement, an oscillating motion shown by the arrows in FIG. 3 is generated at the distal end of the oscillator 1. When viewed from the contact surface (the upper surface of the oscillator), this vibration corresponds to a single travelling wave. When a rotor 2 having a contact spring portion is brought into press contact with the oscillator, the rotor contacts the stator at only one portion located near the wavefront, and is rotated in the reverse direction of the travelling wave. The output is extracted by a gear 4 attached to the outer circumferential surface of a ball bearing 3 at the upper portion of the rotor a.
In the rod-shaped vibration driven motor, a support pin rod 5 (shaft distal end)--flange 6 system is integrated, and the natural mode of the oscillator is FEM-analyzed, so that the vibration amplitude of the flange 6 is minimized. For this reason, the rod-like vibration driven motor has a very small support loss as compared to a ring-shaped oscillator.
On the other hand, a contact portion 7 having a spring structure is formed on the lower portion of a rotor main ring of the rotor 2, and has a natural frequency sufficiently higher than the vibration application frequency of the oscillator as in a ring-shaped oscillator, so that it can follow the driving vibration. The rotor main ring has a sufficiently large inertial mass, so that no vibration is excited by a vibration applied from the oscillator.
One drawback of a vibration driven motor is its low motor efficiency. In general, in a rod-shaped vibration driven motor, since an oscillator is supported by an elastic member using a metal pin consisting of, e.g., stainless steel, the support loss is small. Also, since the oscillator and the rotor consist of a material such as brass, aluminum, or the like, which causes less internal vibration attenuation, their internal losses are also small.
Therefore, a loss due to slippage at the contact portion between the rotor and a stator is the dominant factor of energy losses in such a motor, and the object of this invention is to improve motor efficiency by eliminating this loss factor.
The slippage at the contact portion can be classified into a radial slippage and circumferential slippage in a cylindrical coordinate system having the rotational axis of the rotor as the center. Methods of preventing radial slippage have been disclosed in Japanese Laid-Open Patent Application Nos. 61-224882, 63-174581, and the like. However, no countermeasures are conventionally taken against the circumferential slippage.
A mechanism of the circumferential slip will be described below.
FIG. 4A shows the peripheral speed corresponding to the vibration displacement on the contact surface of an oscillator at a certain time by respective lengths and directions of arrows, and FIG. 4B shows the peripheral speed distribution at that time. As can be seen from FIG. 4B, a vibration formed on the contact surface of the oscillator as a stator has a high speed near the wavefront.
FIG. 5 shows the state of the contact portion of a movable member, such as a rotor, contacting the oscillator. Referring to FIG. 5, a solid waveform represents the displacement of the oscillator, and a broken waveform represents the displacement of the rotor. The respective speeds at two points A and B on the contact surfaces of the oscillator and the rotor may be represented by V.sub.as (the speed of the oscillator at the point A), V.sub.ar (the speed of the rotor at the point A), V.sub.bs (the speed of the oscillator at the point B), and V.sub.br (the speed of the rotor at the point B).
Although the rotor is rotated at a certain peripheral speed v.sub.0 by, e.g., an inertia, since the rotor contact surface contacts the vibration surface of the oscillator over a finite length, the rotor contact surface is fed at the respective contact points at different peripheral speeds v.sub.as to v.sub.bs. Therefore, contact portions corresponding to rotor peripheral speeds other than v.sub.0 cause slippage. For example, slippage of a relative speed v.sub.as -v.sub.0 occurs at the point A, and slippage of a relative speed v.sub.0 -v.sub.bs occurs at the point B.
The product of the slip in the circumferential direction (driving direction) with the frictional force corresponds to a power loss at the sliding portion, and the object of this invention is to reduce or eliminate this power loss.
The reason why the above-mentioned speed difference occurs will be described below with reference to FIGS. 7 to 10.
FIG. 7 shows the radial flexure distribution on a neutral plane (indicated by a dotted curve) N in the contact spring portion formed on the rotor, and FIG. 8 shows the flexural deformation state in the circumferential direction at that time. As shown in FIG. 8, in the contact state with the oscillator, the circumferential distortion of the contact surface in the contact spring of the rotor is shrinkage (the speed is low) near the wavefront to have the neutral plane as the center, and expansion (the speed is high) near a contact release point. FIG. 9 shows temporal transitions of a point position on the contact surface as a vibration wave travel. T represents the time required for a travelling wave to travel one wavelength for one cycle, the vibration wave of the oscillator travels from the left to the right in the plane of the drawing in FIG. 9, and the vibration wave of the rotor travels from the right to the left in the plane of the drawing in FIG. 9.
Referring to FIG. 9, a point P.sub.6 moves from the left to the right at t=1/10T, the entire rotor moves from the right to the left at a peripheral speed v.sub.r, and the central point P.sub.6 of the deformation also has a speed .DELTA.vp.sub.6 due to the deformation. Note that a speed .DELTA.v.sub.4 by the deformation of a point P.sub.4 at t=2/10T has a direction opposite to that of .DELTA.p.sub.6. FIG. 10 shows the speed distribution when the deformation sequentially occurs in this manner. In FIG. 10, the direction of the rotor peripheral speed is defined to be the positive direction.
On the other hand, the peripheral speed distribution on the contact surface of the oscillator has a sine wave state, as shown in FIG. 11, and a difference .DELTA.v between the peripheral speeds of the oscillator and the rotor has a distribution state shown in FIG. 12 in the contact region.